1. Field of the Invention
The present invention relates to a process for the preparation of a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor. More particularly, the invention relates to a process for the preparation of a radiation image storage panel comprising a support, a light-reflecting layer and a stimulable phosphor layer, superposed in this order.
2. Description of the Prior Art
For obtaining a radiation image, there has been recently proposed and practically used a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968. In the method, a radiation image storage panel (i.e., stimulable phosphor sheet) comprising a stimulable phosphor is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with stimulating rays such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, and it is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as a radiographic intensifying screen employed in the conventional radiography.
For enhancing the sensitivity of the radiation image storage panel, there has been known the art that a light-reflecting layer is provided between the support and the stimulable phosphor layer by depositing a metal such as aluminum, etc. on the surface of the support, laminating a metal foil such as an aluminum foil thereon, or applying a coating dispersion comprising a binder and a light-reflecting material thereonto. As for the light-reflecting material, titanium dioxide, white lead, zinc sulfide, aluminum oxide, magnesium oxide and alkaline earth metal fluorohalides are employed as described in U.S. patent application Ser. No. 586,691 now U.S. Pat. No. 4,229,968. A light emitted by the stimulable phosphor in the phosphor layer and advancing towards the support is reflected by said layer and released from the phosphor layer-side surface of the panel. Accordingly, the light advancing towards the support is also detected to enhance the sensitivity of the panel.
However, there is a problem that air bubbles are apt to occur on the interface between the light-reflecting layer and the stimulable phosphor layer in the course of forming them by coating the support successively with a coating dispersion comprising a binder and a light-reflecting material and a coating dispersion comprising a binder and a stimulable phosphor (i.e., successive coating method), and the bubbles affect a resulting image to cause lowering of image quality (unevenness of image density). The occurring of bubbles is presumed to result from that a solvent in a coating dispersion for the phosphor layer permeates the light-reflecting layer when applying the coating dispersion thereonto and makes air dispersively contained in the light-reflecting layer rise to the surface thereof to be concentrated thereon.
There has been proposed a method of simultaneously forming a stimulable phosphor layer and a protective film (or a single phosphor layer which also serves as a protective film in the case of both binders being compatible with each other) on the support by simultaneously coating a binder solution containing a stimulable phosphor and a binder solution containing no stimulable phosphor in a superposed form, as described in U.S. patent application Ser. No. 771,122 now U.S. Pat. No. 4,728,583. This superposition-coating method brings about simplification of a process for the preparation of a panel and enhancement of adhesion between the phosphor layer and the protective film. Further, nonexistence of an adhesive agent therebetween prevents the stimulating rays and the emitted light from being reflected by each interface between the phosphor layer, the adhesive layer and the protective film, to enhance sensitivity and image quality.